This invention pertains to a coal liquefaction process, enhanced with respect to reductions in process performance excursion during changeover in process feedstocks.
A well known process for the liquefaction of coals is the solvent-refining process whereby feed coal is slurried in a solvent, hydrogenated at elevated temperatures and pressures, and solvent de-ashed. Typical of such process is that described in U.S. Pat. No. 4,164,466 wherein the solvent de-ashing stage comprises a number of separation zones.
Also typical is the recycle of one or more streams from solvent de-ash separation zones to function as process solvent. Such a recycle is shown in U.S. Pat. Nos. 4,189,372, 4,119,523, and 4,230,556.
Other typical processes for the liquefaction of coal are the Kerr-McGee process (U.S. Pat. No. 3,607,718), Etchcol (U.S. Pat. Nos. 3,519,555; 3,540,995; 3,700,584; 4,045,329; 3,755,137; 3,856,658; 3,962,070; 4,054,504), Synthoyl (West German Pat. No. 2,700,309), Dow (U.S. Pat. Nos. 4,102,775; 4,136,013 and British Pat. No. 1,596,556), SRC-I (U.S. Pat. Nos. 3,884,794; 3,884,745; 3,884,796; 3,892,654; 4,110,192; 4,190,581), SRC-II (U.S. Pat. Nos. 3,884,796; 4,111,663; 4,312,763; 4,190,518), EDS (U.S. Pat. Nos. 3,726,784; 4,085,031; 4,085,032; 4,085,033; 4,283,267; 4,252,633; 4,125,452; 4,222,844), Chevron (U.S. Pat. Nos. 4,255,248; 4,264,429) and 2-stage liquefaction processes.
In the solvent refining process for coal liquefaction, an effective process solvent must be coal derived to permit continuous plant operation. The solvent must be a distillate with a nominal boiling range of 450.degree.-900.degree. F., must be able to sustain a 20-50 wt. % coal slurry through feed pumps, must rapidly accept coal dissolution products in solution or suspension, must act as a hydrogen donor/shuttling agent for hydrogen transfer under a free radical mechanism, and must carry the liquefaction product stream through solid/liquid separation processes and ultimately be recycled to continue the process.
The chemical properties of recycle solvents in the SRC process have been well documented. Such solvents exhibit a unique molecular composition comprising a highly complex mixture of polynuclear aromatic and hydroaromatic, polynuclear heteroaromatic and hydroaromatic, alkyl (C.sub.1 -C.sub.4) substituted polynuclear aromatic and hydroaromatic, and alkyl (C.sub.1 -C.sub.4) substituted polynuclear heteroaromatic and hydroaromatic compounds. The major molecular species (10-25%) are two ring aromatics or naphthalenes. In the SRC-I process, 0-30 wt. % new solvent material is generated on a MAF feed coal basis per pass of the solvent/coal mixture through the reactor.
It has been found that bituminous coal, when mixed with solvent, begins to dissolve at temperatures of 200.degree.-250.degree. F. The larger portion of the coal matrix, however, remains as a finely divided particulate at these conditions. Primary dissolution products are believed to be small molecules already present and trapped within the feed coal. Depending on coal type and rank, these compounds may represent 5-20% of the feed stock. In the Wilsonville SRC process, a coal feed slurry is held at 150.degree.-250.degree. F. before entering the preheater under a hydrogen partial pressure of approximately 2000 psi. This initial holding period allows the solvent to extract and exchange with the small trapped molecules in the coal and fill other voids created by loss of water and swelling.
Under preheater conditions (2000 PSI H.sub.2 and up to 780.degree. F.), nascent free radicals first radically combine with labile hydrogen from the feed coal to form a cresol soluble product. Hydrogenation reactions are exothermic and drive the preheater temperature upward by 25.degree.-60.degree. F. Under these conditions, the recycle solvent appears to rapidly solvate the primary liquefaction products. On entering the reactor, exothermic reactions drive the temperature to approximately 840.degree. F. during a residence time of 30-60 minutes. Within the reactor and preheater, original recycle solvent and new coal product components act to shuttle hydrogen to free radical sites generated from splitting and cracking reactions.
Therefore, the solvent within the SRC process reactor will contain 0-30 wt. % MAF coal cracking and splitting products. This solvent may then be recycled to continue the process. Alternatively, this solvent may be hydrogenated before recycling or augmented with materials such as Light Solvent Refined Coal (LSRC) or makeup solvent.
The relationship between the characteristics of recycle solvents and process performance has been clearly demonstrated. Such factors as asphaltene and preasphaltene concentration, boiling point distribution, and degree of saturation all markedly effect process performance. Such parameters as conversion rate, oil make, hydrocarbon gas production, hydrogen consumption, and quality of recycle solvent are indicative of overall performance. A particulary important process performance parameter is the consumption of hydrogen within the liquefaction reactors of the SRC process. Excessive hydrogen consumption causes the hydroliquefaction process to become uneconomical and increases the formation of undesirable products.
It has been found that optimal hydrogen consumption can be achieved by optimizing the compatability of hydro-liquefaction process feed coal with process first stage recycle solvent. In the prior art, such compatibility is achieved through the maintenance of consistent feedstock characteristics and by recycle of process solvents produced in steady-state operation with a given feedstock. In a commercial hydro-liquefaction plant, however, such restrictions on feedstock are impractical and uneconomical. Feedstock changeover however, is unavoidable, notwithstanding its detrimental effect on hydrogen consumption and, therefore, on overall process economy.
It is therefore the general object of the present invention to provide a process by which feedstock changeovers may be accomplished without significantly degrading hydro-liquefaction process performance or economy.